The present invention relates to a machining simulation apparatus and method for NC machining and, more particularly, to an apparatus and a method for generating a numerical control command on the basis of a machining simulation performed on the basis of blank stock shape data, tool shape data and data specifying a workpiece shape.
An adaptive control command generating system for a conventional numerical control system will be described with reference to a block diagram in FIG. 1.
An NC program for use in a machining operation to be performed is stored in an NC program memory 11.
An NC program interpreting section 12 reads NC program blocks on a one-by-one basis from the NC program memory 11, and interprets an NC program block to output an interpolation type, a target position and a feed rate specified in the current block or those specified on a modal basis in a previous block to an interpolating section 13.
The interpolating section 13 calculates movement amounts xcex94x, xcex94y, xcex94z along the respective axes per unit time (interpolation cycle period) on the basis of the interpolation type, the target position and the feed rate, and outputs the movement amounts to a servo control section 14.
The servo control section 14 controls the rotations of axial motors on the basis of the movement amounts xcex94x, xcex94y, xcex94z along the respective axes per unit time obtained from the interpolating section 13 to perform axial operations.
A cutting monitoring section 15 receives a spindle load and feed axis loads actually detected by the servo control section 14, and outputs at least one of these loads to an adaptive control section 16.
The adaptive control section 16 compares the spindle load and the feed axis loads obtained from the cutting monitoring section 15 with predetermined values. If the spindle load and the feed axis loads are greater than a predetermined overload judgment value, the adaptive control section 16 issues an alarm and outputs a command for stopping the axial operations to the interpolating section 13. If the spindle load and the feed axis loads fall outside a predetermined rate control detection range, the adaptive control section 16 outputs a rate change command for increasing or reducing a feed rate to the interpolating section 13. If the spindle load and the feed axis loads are lower than a predetermined air-cut judgment value, the adaptive control section 16 outputs a feed rate command suitable for air-cut to the interpolating section 13.
On the basis of these commands, the interpolating section 13 re-calculates the movement amounts xcex94x, xcex94y, xcex94z along the respective axes per unit time (interpolation cycle period), and outputs the re-calculated movement amounts to the servo control section 14.
The aforesaid process is repeated until the machining operation ends.
With reference to a block diagram in FIG. 2, an explanation will be given to a conventional NC program generating system.
An operator inputs information such as a tool type, a tool size, a blank stock material and a machining path required for the generation of an NC program to a machining data inputting section 21. The input result is outputted to an NC program generating section 23.
A cutting condition data table 22 is in a data table form which allows for determination of an optimum feed rate, an optimum spindle rotation speed and the like on the basis of the tool type, the tool size and the blank stock material and the like, and referred to by the NC program generating section 23.
The NC program generating section 23 generates the NC program on the basis of machining data such as the tool type, the tool size, the blank stock material and the machining path outputted from the machining data inputting section 21, and cutting conditions such as the feed rate and the spindle rotation speed read out of the cutting condition data table 22 on the basis of the tool type, the tool size, the blank stock material and the like.
A table data modifying section 24 outputs a command for modification of a relational table indicative of a relationship of the tool type, the tool size, the blank stock material and the like with the feed rate, the spindle rotation speed and the like.
An NC program editing section 25 allows the operator to directly edit the NC program.
In the NC program generating system having the aforesaid construction, the operator selects one of the following three methods for changing the feed rate or the like in the NC program.
The operator specifies the feed rate or the like directly from the machining data inputting section 21. Alternatively, the operator inputs an instruction for modification of the relational table indicative of the relationship of the tool type, the tool size, the blank stock material and the like with the feed rate, the spindle rotation speed and the like in the table data modifying section 24. Alternatively, the operator changes an F-command or the like in the NC program editing section 25.
In the adaptive control command generating system for the conventional numerical control system, machining state information indicative of the spindle load, the feed axis loads and the like is fed back at a high speed but, even if so, a response delay is inevitable because of operating principles. Accordingly, there is no choice but to perform a xe2x80x9cpresentxe2x80x9d control operation on the basis of xe2x80x9coldxe2x80x9d information.
Where a machining operation with a small machining load follows a machining operation with a great machining load, the feed rate is controlled to be reduced even though the actual machining load is small. This results in a reduction in machining efficiency. Where a machining operation with a great machining load follows a machining operation with a small machining load, the feed rate is controlled to be increased even though the actual machining load is great. This results in an overload on a tool, failing to provide a proper surface roughness.
In the conventional NC program generating system, the cutting conditions such as the spindle rotation speed are specified by the operator according to his own judgment, or uniquely determined on the basis of the tool type, the tool size, the blank stock material and the like. Therefore, the cutting conditions are irrelevant to a forced-vibration frequency and a load variation frequency occurring due to interrupted cutting.
This makes it very difficult to continuously perform a cutting operation at an optimum spindle rotation speed or at an optimum cutting speed, leading to early tool wear and deterioration in machining accuracy and surface roughness. Where the interrupted-cutting forced-vibration frequency and load variation frequency, or harmonic frequencies thereof which are integral multiples thereof are close to the natural frequency of a machine, a tool, a jig, a workpiece or the like, chattering of several tens micrometers occurs due to resonance. As a result, periodic wave marks are formed on a surface of a workpiece thereby to deteriorate the surface roughness.
A conventional method to be taken when the chattering occurs is to change the frequency of the interrupted cutting (in general, the spindle rotation speed) so as to prevent the interrupted cutting frequency from being close to the natural frequency of the machine, the tool, the jig, the workpiece or the like.
However, this method heavily relies on a cut-and-try approach. Further, when the chattering is detected during a trial cutting operation, it is necessary to find conditions for elimination of the chattering. This operation is disadvantageously time-consuming even if performed by a skilled operator.
To overcome the aforesaid problem, it is an object of the present invention to provide a machining simulation apparatus and method for NC machining, wherein a machining simulation is performed on a graphic data basis prior to machining, and a spindle rotation speed is reflected on the actual machining and the generation of a machining program under conditions optimum for the actual machining on the basis of an interrupted-cutting forced-vibration frequency and load variation frequency obtained through the machining simulation.
There is provided an apparatus for performing a machining simulation for NC machining on the basis of machining information, the apparatus comprising: machining simulation means for simulating a forced-vibration frequency and a load variation frequency occurring due to interrupted cutting on the basis of the machining information; and numerical control command generating means for generating a numerical control command on the basis of the interrupted-cutting forced-vibration frequency and load variation frequency obtained from the machining simulation means, whereby the aforesaid object is achieved.
The machining simulation is performed on a graphic data basis prior to machining, so that a spindle rotation speed can be reflected on the actual machining and the generation of a machining program under conditions optimum for the actual machining on the basis of the interrupted-cutting forced-vibration frequency and load variation frequency obtained through the machining simulation. Therefore, the interrupted-cutting forced-vibration frequency and load variation frequency, or harmonic frequencies thereof which are integral multiples thereof are prevented from being close to the natural frequency of a machine, a tool, a jig or a workpiece. Thus, chattering can be prevented, thereby improving a surface accuracy.
Further, the machining information includes an NC program, blank stock shape data and tool shape data, whereby the aforesaid object is achieved.
The machining simulation means calculates the interrupted-cutting forced-vibration frequency on the basis of a spindle rotation speed specified in the NC program and a tool tooth number specified in the tool shape data, whereby the aforesaid object is achieved.
The machining simulation means includes: blank stock shape data generating means which converts the blank stock shape data into machining simulation shape data defined by three-dimensional lattice points prior to the simulation; simulation shape storage means for storing the machining simulation shape data; and cutting amount calculating means which calculates a cutting amount indicative of a volume to be removed when a tool passes through a portion of a stock blank represented by the lattice points on the basis of the NC program and the tool shape data, and updates information indicative of presence or absence of lattice points in the blank stock portion through which the tool passes in accordance with the removal, wherein the cutting load variation frequency is calculated on the basis of the spindle rotation speed specified in the NC program, the tool shape data and the cutting amount calculating means, whereby the aforesaid object is achieved.
The machining simulation means may include: blank stock shape data generating means which converts the blank stock shape data into machining simulation shape data by defining a bottom face of a blank stock specified in the blank stock shape data by lattice points and defining a height of the blank stock by vertical height data of each of the lattice points prior to the simulation; simulation shape storage means for storing the machining simulation shape data; and cutting amount calculating means which calculates a cutting amount indicative of a volume to be removed when a tool passes through a portion of the blank stock represented by the lattice points on the basis of the NC program and the tool shape data, and updates the vertical height data of lattice points of the blank stock portion through which the tool passes in accordance with the removal, wherein the cutting load variation frequency is calculated on the basis of the spindle rotation speed specified in the NC program, the tool shape data and the cutting amount calculating means, whereby the aforesaid object is achieved.
The vertical height data includes height data of a solid blank stock portion, or includes height data of a hollow blank stock portion in combination with height data of a solid blank stock portion, whereby the aforesaid object is achieved.
The numerical control command generating means generates an NC program, NC program decode data obtained by decoding the NC program in a numerical controller, or a cutting speed determined on the basis of the NC program decode data so that the interrupted-cutting forced-vibration frequency and load variation frequency obtained from the machining simulation means or harmonic frequencies thereof which are integral multiples thereof fall outside a predetermined range including a natural frequency of the machine, the tool, the jig or the workpiece, whereby the aforesaid object is achieved.
The blank stock shape data is shape data obtained through the machining simulation on the basis of inputted workpiece shape data, or data obtained by correcting a part of the shape data obtained through the machining simulation with the use of shape data obtained by actual measurement on the workpiece, whereby the aforesaid object is achieved.
The tool shape data may be data obtained by correcting inputted tool shape data with the use of shape data obtained by actual measurement on the tool, whereby the aforesaid object is achieved.
Further, there is provided a method for performing a machining simulation for NC machining on the basis of machining information, the method comprising: a machining simulation step for simulating a forced-vibration frequency and a load variation frequency occurring due to interrupted cutting on the basis of the machining information; and a numerical control command generating step for generating a numerical control command on the basis of the interrupted-cutting forced-vibration frequency and load variation frequency obtained in the machining simulation step, whereby the aforesaid object is achieved.
In the numerical control command generating step, an NC program, NC program decode data obtained by decoding the NC program in a numerical controller, or a cutting speed determined on the basis of the NC program decode data is generated so that the interrupted-cutting forced-vibration frequency and load variation frequency obtained from the machining simulation means or harmonic frequencies thereof which are integral multiples thereof fall outside a predetermined range including a natural frequency of a machine, a tool, a jig or a workpiece, whereby the aforesaid object is achieved.
To achieve the aforesaid object, there is provided a medium storing thereon a program, which causes a computer to perform a machining simulation step for simulating a forced-vibration frequency and a load variation frequency occurring due to interrupted cutting on the basis of machining information, and a numerical control command generating step for generating a numerical control command on the basis of the interrupted-cutting forced-vibration frequency and load variation frequency.
By the program for performing the machining simulation step, an NC program, NC program decode data obtained by decoding the NC program in a numerical controller, or a cutting speed determined on the basis of the NC program decode data is generated so that the interrupted-cutting forced-vibration frequency and load variation frequency or harmonic frequencies thereof which are integral multiples thereof fall outside a predetermined range including a natural frequency of a machine, a tool, a jig or a workpiece.